Biology and Control

of Hemlock Woolly Adelgid

FHTET-2014-05

April 2014

Nathan P. Havill

Lígia C. Vieira

Scott M. Salom

Technology

Transfer Non-native Pest

Forest Health Technology 

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On the cover:

Background image: Hemlock mortality, Jason Van Driesche, Bugwood.org

Bottom left to right: HWA white ovisacs on eastern hemlock branch , Scott M. Salom, 

Virginia Tech; Sajiscymus tsugae, Carol Cheah, Bugwood.org; Laricobius osakensis, 

Ligia C. Vieira, Virginia Tech.

Pesticide Precautionary Statement 

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been registered. All uses of pesticides must be registered by appropriate State and/or Federal 

agencies before they can be recommended. CAUTION: Pesticides can be injurious to 

humans, domestic animals, desirable plants, and fish or other wildlife--if they are not handled or 

applied properly. Use all pesticides selectively and carefully. Follow recommended practices for the 

disposal of surplus pesticides and pesticide containers.

CAUTION:

PESTICIDES
Preface

This publication is a substantial revision of FHTET 2001-03, Hemlock Woolly Adelgid, which was 

published in 2001. This publication contains information on the native range of hemlock and range of 

hemlock woolly adelgid, the importance of hemlocks in eastern forest ecosystems, and on hosts, life 

cycle, control, and population trends of the hemlock woolly adelgid.

For copies of this publication or information concerning hemlock woolly adelgid, contact Dr. Scott 

Salom in Blacksburg, VA at 540-231-2794, Dr. Richard Reardon in Morgantown, WV at 304-285-1566 

or the local Cooperative Extension Service Office. The entire publication is available online at 

http://www.fs.fed.us/foresthealth/technology/pub_programareas.shtml

The authors thank Denise Binion for editing, layout and design and the U.S. Department of 

Agriculture, Forest Service, Forest Health Protection, Northeastern Area, Region 8 and the Forest 

Health Technology Enterprise Team for providing funding for the preparation and printing of this 

publication.

Biology and Control

of Hemlock Woolly Adelgid

Nathan P. Havill

USDA Forest Service, Northern Research Station, Hamden, CT

Lígia C. Vieira

Virginia Polytechnic Institute and State University, Blackburg, VA

Scott M. Salom

Virginia Polytechnic Institute and State University, Blackburg, VA

FHTET-2014-05

April 2014

Table of Contents

Introduction ....................................................................................................1

Hosts and Spread in Eastern North America..............................................2

Importance of Hemlock .................................................................................4

Life Cycle of Hemlock Woolly Adelgid.........................................................5

Feeding .............................................................................................................7

Impact on Hemlock ........................................................................................9

Factors Regulating HWA Populations .......................................................10

Host Plant Resistance and Tolerance.........................................................10

Genetic Resource Conservation.................................................................11

Chemical Control ..........................................................................................11

Biological Conrol ..........................................................................................13

Toward Integrated Pest Management of HWA .......................................16

References .....................................................................................................17

________________________________________Biology and Control of Hemlock Woolly Adelgid

 1

Introduction

The hemlock woolly adelgid (HWA) (Adelges tsugae Annand) (Figure 1) is a destructive, non-native 

pest of forest and ornamental hemlock trees in eastern North America. As a member of the Adelgidae, 

it shares many characteristics with its relatives, the true aphids (Aphididae) and phylloxerans (Phylloxeridae), such as a complex life cycle with sexual and asexual generations. Adelgids are different 

from true aphids in that they are oviparous (lay eggs) in all generations and use only certain conifer 

genera as hosts. The Adelgidae also includes other invasive pests such as the balsam woolly adelgid, 

A. piceae (Ratzeburg), on true firs in North America, and the pine adelgids, Pineus pini (Macquart) 

and P. boerneri Annand, in pine plantations around the World.

HWA inserts its long, piercing-sucking mouth parts (Figure 2) into the base of hemlock needles and 

feeds on nutrients stored in the xylem ray cells. In addition to removing the tree’s nutrients, the adelgid is thought to cause a hypersensitive response that restricts water transport in the tree. Other stress 

factors can intensify these effects, causing decline and death of trees of all age classes. 

The first record of HWA in the eastern United States is from museum specimens collected in 1951 in 

Richmond, Virginia. It arrived there some time prior to this date, directly from southern Japan, probably on live plant material. The HWA genotype that was introduced to the United States is one of several, very different lineages found on hemlocks in Asia. In addition to the four known Asian lineages 

(mainland China, Taiwan, and two in Japan), there is a distinct lineage in western North America, 

Figure 1. HWA on eastern hemlock (Image: Scott M. Salom, Virginia Tech).
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Biology and Control of Hemlock Woolly Adelgid _______________________________________

which is now known to be native to that region. The first records of HWA in the West date from as 

early as 1916 in British Columbia, but these records have no relevance to the introduction in the East 

because it is a different lineage. Introduction of additional genotypes which are native to different 

places could worsen the pest problem in the eastern United States. 

The HWA genotype that was introduced to the eastern United States from southern Japan can alternate between southern Japanese hemlock, T. sieboldii Carrière, and tiger-tail spruce, Picea torano (K. 

Koch) Koehne. On spruce, there is a sexual 

generation and gall formation. When this 

spruce species is not present, HWA can have 

continuous cycles of asexual generations 

on hemlock. This is the type of life cycle 

that the Japanese adelgids exhibit in eastern 

North America because they were separated 

from their primary host when they were introduced. The western North American adelgids also have an asexual life cycle, but their 

separation from a suitable spruce host may 

have happened millions of years ago when 

they dispersed naturally from Asia.

In Japan, HWA can occasionally reach high 

densities on individual trees, but it is very 

rare that it causes damage. This is probably 

due to a combination of host resistance, host 

tolerance, and a diverse complex of arthropod predators that seems to regulate HWA 

populations.

Hosts and Spread in Eastern North America

In eastern North America, HWA is a destructive pest of eastern hemlock, Tsuga canadensis (L.) 

Carrière, and Carolina hemlock, T. caroliniana Engelman. Both species have shown very limited 

resistance to HWA feeding, and many trees growing under a wide variety of natural and ornamental 

conditions have died.

The geographic range of eastern hemlock extends from the Canadian Maritime Provinces, west to the 

Great Lakes region, and as far south as northern Georgia and Alabama, with many separated populations to the south and west of its main range (Figure 3). Carolina hemlock has a much smaller range 

with isolated pockets in Virginia, North Carolina, South Carolina, Tennessee, and Georgia.

Currently, HWA occupies approximately half of eastern hemlock’s natural range (Figure 4). The initial spread was predominantly to the North, but this has slowed as HWA reaches colder areas. To the 

South, spread is very rapid and HWA has almost filled the entire southern range of hemlock. New, 

isolated range expansions associated with movement of ornamental hemlock trees in places such as 

Michigan and southern Ontario are met with vigorous eradication efforts. 

Figure 2. Slide-mounted HWA adult (left) and first instar (right) (Photos: 

Nathan Havill, USDA Forest Service).
________________________________________Biology and Control of Hemlock Woolly Adelgid

 3

Since the HWA lineage that arrived in eastern North America from Japan is adapted to T. sieboldii, a 

species limited to southern Japan at low elevations, it is expected that HWA would not survive as well 

in colder areas. This appears to be the case, as tree mortality is occurring at a slower pace in northern 

New England, while in the South the decline has been more rapid.

Figure 3. Geographic distribution of the four North American hemlock species — Tsuga caroliniana

(red), T. canadensis (yellow), T. mertensiana (blue) and T. heterophylla (green). Redrawn from maps in 

USGS (1999).
4 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

Importance of Hemlock

Hemlocks are coniferous trees in the family Pinaceae, belonging to the genus Tsuga Carrière. The 

number of species described in this genus has changed over time, but contemporary authors generally 

accept ten species. The species native to eastern North America are eastern hemlock (Tsuga canadensis (L.) Carrière) and Carolina hemlock (Tsuga caroliniana Englem.). 

There is a general recognition that hemlock forests have great ecological value, as they are frequently 

considered a foundation species in areas not recently disturbed. Foundation species are tree species 

that define the forest structure and control ecosystem dynamics. Hemlocks commonly grow along 

headwater streams, greatly influencing stream characteristics like temperature and nutrient cycling, 

as well as their biotic communities. Hemlocks have such a distinct influence on freshwater dynamics 

that streams flowing through hemlock forests support unique assemblages of salamanders, fish, and 

freshwater invertebrates that are intolerant of seasonal drying. Several wildlife species depend on the 

environment that exists in hemlock stands. Some, like the black-throated green warbler (Dendroica 

virens) and the blue-headed vireo (Vireo solitarius), are only present in forests with hemlocks (hemlock obligates).

Hemlocks are widely appreciated for their aesthetics. Their grandiosity and gracefulness captivated Pennsylvania into making it their State Tree in 1931. Hemlocks are commonly planted as a tree, 

shrub, or hedge in ornamental landscapes in the United States and Europe. There are at least 274 culFigure 4. Map of eastern hemlock distribution and range of HWA. (2013)
________________________________________Biology and Control of Hemlock Woolly Adelgid

 5

tivars of eastern hemlock, making it one of the most cultured landscape tree species. The recreational 

appeal of hemlock forests is evident by the millions of people that visit hemlock stands in National 

and State Parks, recreation areas, and private lands in the eastern United States every year.

The economic value of hemlocks as timber and pulp is considerably low. However, there has been 

an effort to quantify the services provided by hemlock forests, as these resources are being threatened 

and forest managers need this information to evaluate the cost and benefits of treatment. Ecosystem 

services performed by hemlock forests have been valued at $969 per hectare per year. Several studies 

have also been conducted to determine how much people would be willing to pay to protect hemlock 

forests. Generally, these studies found that the value attributed to hemlock forests greatly exceeded 

the cost of controlling for exotic species. Additionally, it has been found that in an urban environment, good tree cover (between 40-60%) around a property has been associated with 0.5 to 6% higher 

property values.

Lifecycle of Hemlock Woolly Adelgid

Hemlock woolly adelgid has a complex life cycle that is typical of the Adelgidae (Figure 5). In Japan, 

HWA can alternate between Tsuga sieboldii and Picea torano in areas where these species coexist. 

Picea is considered the primary host because that is where sexual reproduction occurs. Tsuga is the 

secondary host because it supports only asexual generations. There are two asexual generations per 

year on hemlock with a proportion of the second generation developing into winged migrants, called 

sexuparae (Figure 6). When sexuparae land on a suitable spruce, they produce a very short generation 

of sexual females and males. Each mated female lays a single, relatively large egg, which develops 

into an asexual female (fundatrix) that settles near a bud to overwinter. In the spring, feeding by the 

fundatrix begins transforming the bud into a gall that will house her offspring. The fundatrix lays 

a large clutch of eggs. Upon egg hatch, crawlers walk into the developing gall. These individuals, 

called gallicolae, feed inside the multi-chambered gall causing the completion of gall formation. 

HWA galls have a characteristically round shape, unique among adelgid species (Figure 7). A single 

gall can house over 1,000 adelgids. When the winged gallicolae mature, they emerge from the gall as 

it dries, and fly back to hemlock.

Figure 5. Multi-generation life cycle of HWA in its native range in Japan and its introduced range in eastern North 

America (Diagram: Vince D’Amico and Nathan Havill, USDA Forest Service.)
6 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

In North America and in areas of Japan where P. torano is not present, HWA completes a truncated life 

cycle. In this case, only the two asexual generations on hemlock are repeated each year because the 

offspring of migrant sexuparae do not survive. Sexupara production can therefore be viewed as a mortality factor in these areas. HWA is apparent on hemlock by the large, white, woolly ovisacs (Figure 1). 

The first generation is made up of sistentes (singular = sistens, from the Latin “to halt”), so-called because they have a long diapause in the first instar. The next generation is made up of individuals called 

progredientes (singular = progrediens, from the Latin “to proceed”) that do not have a long diapause. 

As a consequence, the progredientes have 

a shorter generation time than the sistentes. 

Progrediens crawlers hatch in the spring and 

search for a suitable place to settle, mostly 

on the previous years’ growth. They disperse 

within and between trees by crawling, being 

blown by the wind, or being transported by 

birds, deer, or other animals. They nearly 

always settle at the base of the needle, where 

they remain attached for the rest of their 

lives. The proportion of progredientes that 

become winged sexuparae increases with 

adelgid density. The wingless progredientes 

that remain on hemlock develop quickly to 

the adult stage and lay a smaller clutch of sistens eggs. The sistens crawlers settle, mostly 

on that year’s new growth when available, 

and experience a long diapause in the first 

instar lasting through the summer. Development resumes in the autumn and progresses 

slowly through the winter until they reach the 

adult stage and lay eggs in late winter/early 

spring. Although the developmental stages 

and generations on hemlock in Japan and 

North America are the same, the phenology 

varies with elevation, latitude, and weather 

conditions. For example, the sistentes generally begin laying eggs in March in New 

England, compared to February in Virginia. 

In any case, there is enormous overlap of life 

stages during the spring and summer months, 

in part due to the long periods of egg laying 

and hatching.

Figure 7. HWA gall on tiger-tail spruce in Japan (Image: Shigehiko Shiyake, Osaka Museum of Natural History.

Figure 6. HWA sexupara, the stage that migrates from hemlock to 

spruce in Japan (Image: Nathan Havill, USDA Forest Service.)
________________________________________Biology and Control of Hemlock Woolly Adelgid

 7

Feeding

After hatching from the eggs, the crawlers search for a suitable 

site to insert their feeding stylets into the plant tissue (Figure 

8). The crawler has prominent bristles on its antennae, legs, 

and mouth parts that are probably sensory organs that help 

locate a suitable settling site (Figure 9). HWA has a stylet 

bundle, which is located on the underside of the insect and 

composed of four individual stylets (Figure 10, arrow). The 

stylets fit together to form two canals: one injects saliva into 

the tree, and the other transports nutrients from the tree to the 

insect (Figure 11). The stylets are typically inserted into the 

stem near the base of the needle, at the point where the waxy 

cuticle of the needle cushion is thinnest. Occasionally, when 

populations are very high, a few HWA individuals can settle on 

the woody part of the stem and survive to reproduce. 

HWA inserts its stylets through the epidermal cells at the plant 

surface to reach the vascular tissue. The stylet bundle is more 

than three times the length of the first instar adelgid (Figure 

2), and penetrates deep within the plant tissue (Figure 12). 

Once the stylets have reached the vascular tissue, they travel 

along the vascular bundle in the xylem, passing between 

cells until they terminate in the parenchyma cells of the 

xylem rays. These cells have the function of transferring and 

storing plant nutrients in the tree and contain large amounts of 

carbohydrates. 

The adelgid probes the tree tissue with its stylet bundle, 

sometimes pulling back and then inserting along a different 

path. While doing so, it secretes saliva within the plant tissue. 

Some components of the saliva harden around the stylets 

and form a salivary sheath, made up mostly of proteins. At 

each molt, the stylet bundle is retracted from the tree and the 

cuticle is shed. After the new cuticle has hardened, the stylet 

bundle is reinserted. The xylem ray parenchyma cells are the 

feeding site for all life stages of the insect. Thus, HWA does 

not deplete nutrients directly from the sap, but removes food 

reserves from the tree’s storage cells.

HWA has up to five different types of endosymbiotic bacteria 

living inside its body, one of which is found in the salivary 

glands of first instars. Some of these bacteria are necessary 

for adelgid survival and probably serve a purpose related to 

nutrition.

Figure 8. HWA nymphs settled and feeding at 

the base of needles.

Figure 9. Ventral view of an HWA crawler. 

Scale bar = 100μm. ant, antenna; la, labium 

(Photo: Kelly Oten, NC State University).
8 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

Figure 10. HWA nymph with stylet bundle (arrow) inserted into base of needle. Arrowhead indicates point of stylet 

insertion.

Figure 11. Cross-section of hemlock woolly adelgid stylets. FC, food canal; Mds, 

mandibular stylet; Mxs, maxillary stylet; NT, neurotubules of mandibles; SC, salivary canal. Scale bar= 1μm (Photo: Kelly Oten, NC State University).

Figure 12. Hemlock needle torn away from stem shows position 

of nymph (arrow) and its stylet bundle (arrowheads) within the 

plant.

FC
________________________________________Biology and Control of Hemlock Woolly Adelgid

 9

Impact on Hemlock

Eastern and Carolina hemlocks are highly susceptible to damage 

from HWA feeding. Symptoms consist of discoloration, desiccation, loss of the needles, and dieback of branches. The feeding 

impact of HWA results in a depletion of the stored nutrients 

which compromises the tree’s ability to produce new growth. 

Recent evidence also suggests that HWA feeding causes a hypersensitive response around colonized tissue causing the induction 

of ‘false growth rings’ in infested stems that interfere with solute 

transport and prevent the stems from obtaining the necessary water for photosynthesis. 

As the infestation progresses, crown vigor decreases (Figure 

13), eventually leading to branch dieback and death of the tree. 

HWA can be fatal to hemlocks of all ages in 4-15 years, depending on moisture availability and other stress factors. In some 

areas, mortality due to HWA infestation has been as high as 95% 

(Figure 14). In other areas, especially in the North, where winter 

temperatures help regulate adelgid survival, hemlock trees can 

continue to survive in a weakened state for more than 15 years 

following the initial infestation. 

Figure 13. A mature eastern hemlock in decline following its infestation from HWA (Photo: Bugwood.

com)

Figure 14. A stand of dead eastern hemlock trees in the Great Smoky Mountains National Park (Photo: 

Scott M. Salom, Virginia Tech.)
10 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

Initially, adelgid populations will be found in the canopy in aggregated distributions. As HWA populations increase on a tree and in a stand, their distribution becomes more uniform to the point where 

virtually all of the new or current year’s shoots become infested. This level of infestation leads to a 

reduction in shoot growth, which in turn leads to a crash in the HWA population. Since the sistentes 

prefer new growth, the density of adelgids will tend to follow tree vigor. The reduced abundance of 

new growth and acceptable feeding sites consequently leads to a decline in HWA populations. Often, 

trees are eventually impacted to the point where they cannot recover.

The loss of eastern hemlock due to HWA has led to immediate, short- and long-term changes in ecosystem structure and function. Initially, hemlock death results in increased light penetration to the 

forest floor and carbon and nutrient inputs in soil and streams, which in turn impacts the flora and 

fauna taking advantage of the increased resources. In many locations, eastern hemlock does not naturally re-establish following mortality due to HWA. It is often replaced by a different ecosystem type 

altogether, dominated by species such as birches (Betula spp.), oaks (Quercus spp.), and maples (Acer

spp.), and by Rhododendron in riparian areas or yellow poplar (Liriodendron tulipifera L.) when 

Rhododendron is absent. This change in ecosystem type impacts ecosystem processes, and leads to 

the loss of terrestrial arthropods, amphibians, plants, fish, and aquatic invertebrate species associated 

uniquely with hemlock forests.

Factors Regulating HWA Populations

Low winter temperature is the most prominent mortality factor controlling HWA populations and limiting its spread in the eastern United States. However, recent studies have called attention to: 1) the 

effects of climate change, which would make previously unsuitable habitats, suitable, and may lead to 

the expansion of this pest’s distribution; and 2) the possibility that HWA is adapting to conditions in 

the introduced range, exhibiting a greater tolerance to colder temperatures. 

Tree health also has an impact on HWA populations. The amount of time a hemlock tree can withstand HWA feeding can be affected by several environmental variables. Water stress has been consistently associated with a faster decline in tree health. Latitude and longitude are also correlated with 

HWA infestation as it relates to the infestation front; that is, the risk of a hemlock stand becoming 

infested is correlated with the proximity of infested stands nearby. Other landscape characteristics 

such as elevation, slope, aspect, and terrain shape index have also been found to be relevant, but less 

consistently. HWA also seems to be regulated in part in a density-dependent manner, where a high 

density of progrediens crawlers negatively impacts the fecundity and survival of subsequent generations. This effect could be mediated by HWA impacts on tree health.

Host Plant Resistance and Tolerance

While the eastern and Carolina hemlocks were found to be highly susceptible to HWA, the Chinese 

hemlock (T. chinensis (Franch.) E. Pritz.) was found to be highly resistant to the pest. Attempts have 

been made to create resistant hybrids between eastern North American and Asian hemlock species 

that retain as much as possible the characteristics of the native species. Crosses between Carolina and 
________________________________________Biology and Control of Hemlock Woolly Adelgid

 11

Chinese hemlocks have resulted in hybrids with intermediate resistance to HWA. However, crossing 

Chinese and eastern hemlock has not been successful.

Reports of individual hemlock trees and stands surviving in areas otherwise devastated by HWA have 

raised doubts about the complete absence of innate resistance in eastern and Carolina hemlocks. In 

order to guide the search for resistant trees, characteristics that might confer resistance have been 

studied. Terpenoid volatiles, foliar chemistry, and physical and chemical characteristics of the needle 

cushion have been associated with resistance. All of these characteristics vary within eastern and 

Carolina hemlock populations, and the foliar chemistry has been associated with some level of resistance in these species.

Foliar concentrations of calcium, potassium, nitrogen, and phosphorous were found to be strongly 

correlated with HWA densities. Higher nitrogen and potassium concentrations were associated with 

higher HWA densities, while higher calcium and phosphorous concentrations were associated with 

lower densities. Nitrogen is a limiting nutrient for most plants and animals. Nitrogen fertilization 

increases tree health, which can make it more capable of withstanding attacks, but also increases the 

nutritional quality of the tree for the pest. 

Genetic Resource Conservation

In the event of the worst-case scenario where the management of HWA fails and eastern and Carolina 

are eliminated from most of their range, there is a program of gene conservation with seed collections 

and ex situ conservation. The first efforts focused on Carolina hemlock due to its smaller population 

size and the fact that HWA has rapidly impacted all of its geographic range. Seed collections throughout its range were completed in 2006, and the genetic variability in the seed samples was found to 

adequately reflect the genetic variability in natural stands of Carolina hemlock. Seed collections for 

eastern hemlock have been ongoing since 2005, and efforts were initially focused on the southern 

part of the species’ geographic range due to higher levels of genetic diversity and the greater threat 

the species faced from HWA in this region. Seed collections have subsequently expanded across the 

entire geographic range of eastern hemlock in the United States, and now include interior populations 

in the Southern Appalachian, Mid-Atlantic, Northeastern, and Upper Midwest regions, and disjunct 

populations that occur to the east and west of main species distribution. At this time more than 2.5 

million hemlock seeds have been placed into conservation, either stored in seed banks at the USDA 

National Center for Genetic Resources Preservation in Fort Collins, CO, the USDA Forest Service 

Ashe Seed Facility in Brooklyn, MS, and at North Carolina State University in Raleigh, NC, or planted into conservation seed orchards in Brazil, Chile, and the United States.

Chemical Control

Foliar insecticides were initially used with good efficacy, but the need for frequent re-application and 

to completely drench the foliage has diminished their use. In the early 1990’s a new class of systemic 

insecticides, the neonicotinoids, became available. Several active ingredients (imidacloprid, dinotefuran, acetamiprid, and thiamethoxam) can successfully control HWA. Due to their systemic activity, 

neonicotinoids can be applied by soil drench, soil injection, and several methods of trunk injection. 

Since xylem sap flow is essential for transporting and distributing the insecticide from the soil to the 
12 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

canopy of the tree, treatment should be applied prior to severe damage to the trees. In healthy trees, 

it can take weeks to several months for the insecticide to be transported to where the adelgid feeds. 

In damaged trees, it can take longer because they have more difficulty transporting the insecticide. 

Treatment should be timed for spring or fall when water uptake by the trees is greatest. Soil and trunk 

application of imidacloprid were found to take 1-3 months to control HWA, while both dinotefuran 

and thiamethoxam were found to be faster. Residual activity of up to three years was observed for 

soil applications of imidacloprid, thiamethoxam, dinotefuran, and acetamiprid. Foliar applications of 

the neonicotinoids provide faster control, but soil applications provide longer residual activity.

In a large scale forest setting, chemical control as a ‘stand-alone’ treatment is not viable, due to the 

costs involved and the potential environmental consequences. Imidacloprid was found to affect other 

canopy feeding arthropods and arthropods inhabiting the soil around the base of the tree. In addition, 

secondary pest outbreaks of spruce spider mites and hemlock rust mites have also been associated 

with imidacloprid treatments for the control of HWA. Considering the close association of eastern 

hemlock with bodies of water, the possibility of insecticide leaching has to be considered as well. 

Special care should be used when applying imidacloprid in areas with soil types that may not bind 

imidacloprid as tightly (e.g., low organic matter content). An optimization of the dosage for each tree 

and adoption of newer tablet formulations should also minimize the risk of contaminating aquatic resources.
________________________________________Biology and Control of Hemlock Woolly Adelgid

 13

Biological Control

With no specialist natural enemies present in eastern North America, HWA is able to flourish and 

spread unchecked. Searching for and studying natural enemies in western North America and parts of 

Asia with native lineages of HWA is considered critical. The primary objective of biological control 

is to introduce a group of natural enemies considered important in the native habitat and deemed safe 

for introduction into the eastern United States. Since no known parasitoids are associated with Adelgidae, the classical biological control program for HWA has focused on prey-specific predator species 

and entomopathogens.

Fungal Pathogens

Twenty fungal genera and 79 entomopathogenic fungus isolates were found in association with HWA 

in the eastern United States and southern China. Of all the isolates, no putative specialists were 

found. A combination of generalist fungi that included two strains of Beauveria bassiana Balasamo, 

one of Lecanicillium lecanii Zimmermann, and one of Metarhizium anisopliae Metchnikoff, demonstrated high efficacy against HWA. A commercially available Lecanicillium lecanii formulated with a 

whey carrier has been tested in small-scale forest trials and has been shown to be effective, especially 

against early instar HWA before they produce wool. This augmentative approach is still in the early 

developmental stages.

Predators

The first efforts toward the application of biological control of HWA in eastern North America started 

in 1992. Of the predators first collected from Japan, the most promising at the time, Sasajiscymnus 

tsugae (Sasaji and McClure) (Coleoptera: Cocinellidae) was imported, evaluated, mass reared, and 

cleared for release (Figure 15). Field releases started in 1995 in Connecticut and more than two million S. tsugae have been released throughout the introduced range of HWA. Post-release evaluations 

indicated successful overwintering, reproduction, and dispersal of S. tsugae, but establishment of the 

predator and impact on HWA populations has been inconsistent. In the South where establishment has 

been closely monitored, the predator population takes 5 to 7 years to reach a detectable level at 

Fig 15. Sasajiscymnus tsugae adult (left) and late instar larva (right) (Photos: Carole Cheah, Bugwood.com.)
14 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

only some of the locations where releases were made. Large-scale rearing of this insect for release is 

being phased out, yet monitoring its presence and impacts will continue.

Additional foreign explorations for natural enemies were carried out from 1995 until 1997 in Yunnan, Sichuan, and Shannxi provinces in China. Three previously unknown lady beetles (Coleoptera: 

Coccinellidae), Scymnus camptodromus Yu et Liu, Scymnus sinuanodulus Yu et Yao, and Scymnus 

ningshanensis Yu et Yao, were found to be the most abundant predators in different locations. The 

predators were imported, evaluated in quarantine, and cleared for release. However, due to difficulties in mass rearing, releases of S. ningshanensis and S. sinuanodulus were delayed until 2004, and 

S. camptodromus was not released. There were no recoveries of the first two species following their 

limited releases, and work continues on rearing of S. camptodromus.

Laricobius beetles in the family Derodontidae are specialist predators of adelgids. In 1997, a predator 

native to western North America, Laricobius nigrinus Fender, was found to consistently feed only on 

HWA in the lab, and its life cycle was shown to be synchronous with HWA (Figure 16). Oviposition 

and subsequent larval development coincides with oviposition by the HWA sistens adults. Females 

oviposit within HWA ovisacs from January to March. After hatching, larvae go through four instars 

and mature larvae drop to the soil to pupate within two weeks. Eclosed adults remain in the soil in a 

state of aestival diapause that coincides with diapausing first instar sistentes. Adults resume activity 

in the fall coinciding with resumption of development by HWA.

Following host-range testing studies in quarantine, the predator was cleared for release in 2000. Rearing procedures were developed and releases began in 2003. To date, more than 200,000 beetles have 

been released at over 200 sites. Evaluations after release showed that establishment is positively correlated with minimum winter temperature and with the number of beetles released. Beetles initially 

disperse slowly up the release tree and then away from the release tree at a rate of close to 40 m/yr. 

Some anecdotal evidence suggests this could be greater at times. 

Due to the difficulty in rearing L. nigrinus, only 500 beetles were available for each release in forested settings. As rearing has improved and beetles can now be collected and redistributed from already 

established sites, the numbers being released have increased to range from 750 – 2,000 beetles per 

site.

Laricobius nigrinus collected from coastal sites in the West (coastal strain) appear to establish better 

in warmer climates (plant hardiness zones 6a and higher). Laricobius nigrinus from Idaho and Montana (interior strain) that are better adapted to colder temperatures are being reared and are beginning 

to be released in the more northern range of introduced HWA. 

Fig 16. Laricobius nigrinus adult and its yellow egg (left) and late instar larva with HWA eggs (right) (Photos: Rob Flowers, Virginia Tech).
________________________________________Biology and Control of Hemlock Woolly Adelgid

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A native eastern North American Laricobius species (Laricobius rubidus Le Conte) has also been 

found on HWA infested hemlock. This predator feeds mainly on native pine bark adelgid, Pineus 

strobi Hartig. It prefers this prey when given a choice, but is able to feed and develop on HWA. It is 

also active from fall through mid-spring and there is a considerable overlap of feeding by both beetle 

species. Laricobius nigrinus and L. rubidus have been found to interbreed at sites where L. nigrinus

was released in the eastern United States. Research indicates that at these sites, L. nigrinus and L. rubidus are hybridizing at a steady rate (ca. 10 - 15%) and L. nigrinus over time may reduce the proportion of L. rubidus on hemlock, but not on white pine at sites where both tree species are present.

Because the strain of HWA that was introduced to eastern North America is from southern Japan, an 

additional effort was made to look for natural enemies in this location. A new species, Laricobius 

osakensis Montgomery and Shiyake, was discovered in 2005 and imported to quarantine to study its 

biology and host-range (Figure 17). The insect was also studied in its native habitat. Laricobius osakensis has host preferences similar to L. nigrinus, greater fecundity, and a higher predation rate then L. 

nigrinus. Lab studies showed that it cannot hybridize with the North American Laricobius species. 

In its native habitat, it is shown to be the key natural enemy on sistentes, being consistently present 

in association with HWA, and having a synchronous life cycle with HWA. Its population fluctuations 

were correlated with those of the HWA sistens population, and it was shown to reduce populations of 

HWA on T. seiboldii. Permission to remove L. osakensis from quarantine was received in 2010 and 

the first open releases began in 2012. Continued efforts toward large-scale operational releases and 

monitoring of this predator are underway.

Fig 17. Laricobius osakensis adult female with typical reddish brown coloration (upper left), adult male with 

typical dark coloration (upper right), and larva (bottom center) (Photos: Ligia C. Viera, Virginia Tech).
16 

Biology and Control of Hemlock Woolly Adelgid _______________________________________

Additional predators from western North America that could target the progrediens generation include 

Scymnus coniferarum (Crotch) (Coleoptera: Coccinelidae), found in association with HWA in Washington, and silver flies (Leucopis spp.: Diptera: Chamaemyiidae) whose larvae and adults were found 

to be positively correlated to HWA densities in Oregon and Washington. Work is ongoing to determine their viability as biological control agents.

Toward Integrated Pest Management of HWA

Researchers working in open-grown hemlock plantations in the southern United States have noticed 

that trees growing under full sunlight do not appear to support dense populations of HWA. Trees in 

the shade and under forest canopies, or a tree’s lower branches shaded by upper branches, tend to be 

colonized more readily by HWA. At this time we do not know if physical differences in these young 

trees with elongated shoot growth or direct radiation and increased temperature from direct exposure 

to the sun for long durations are factors in reducing HWA survival. Regardless of the precise mechanism, it is useful to know that under certain conditions, trees may be less susceptible to HWA colonization and impact. Further investigations can test whether thinning stands to expose hemlock trees 

to more sunlight can be used to reduce HWA infestations and subsequent impacts on hemlock health, 

keeping in mind that sudden exposure to full sun could damage the shallow roots of hemlock trees. 

It has recently been proposed that the integration of chemical and biological control on an area-wide 

scale may help save some hemlocks that otherwise would not survive in the long-term by any one 

control method used alone. The concept currently being tested involves the chemical treatment of 

a subset of mature infested hemlock while releasing predators on understory trees. The chemical 

treatment could provide short-term protection for the larger trees and allow the predator populations 

to build up in the understory. Once the larger trees lose their chemical protection, an established 

predator population might be more capable of controlling a building HWA population than a predator 

population still on the process of becoming established. These kinds of integrated approaches should 

continue to be evaluated while new management tools are developed for HWA control.
________________________________________Biology and Control of Hemlock Woolly Adelgid

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